The present invention relates to a method and apparatus for controlling the absorptance of an electrochromic layer.
Electrochromic layers become dark (colored) in the presence of an applied voltage or when a charge is supplied, i.e., their absorptance increases. Upon the removal of charges, the layers become transparent (decolored) again.
It is known to make use of this phenomenon for display elements, in which case only the transparent and the colored states are used. In this connection, for the coloring process a defined current is fed for a defined period of time. Too great a supply of charges must be avoided, to avoid destruction of the electrochromic layer. On the other hand, the decoloring process is not critical, since it is not possible to remove from the layer more charges than are present in it, and the completely charge-free layer is also the layer with the lowest absorptance. One substantial advantage of electrochromic layers is that they consume current only during a change in their absorptance.
For display elements, two states of the electrochromic layer are sufficient. For other purposes of use, such as sunglasses, optical filters and dimmable mirrors, however, intermediate values are advantageous or necessary. If identical charge and discharge times with identical charge and discharge currents (i.e., identical supply and removal of charges) are used for shifting between such intermediate values, then the layer becomes brighter and brighter (progressively more transparent), since a greater increment of charge is required to effect a given increment in the coloring of an electrochromic layer than is the charge decrement (charge removal) required for the corresponding decrement of coloring (decoloring).